This invention relates to mold powders employed in ingot steel production. More specifically, the present invention discloses a casting powder to be employed in bottom pour steel molds which has the unique ability to act as both a bottom pour flux and a hot topping compound.
The use of bottom pour process to produce ingot steel has enjoyed substantial recent success with millions of tons of steel each year. Produced with this process. In order for the process to work effectively, fluxes must be added on the surface of the molten steel as it begins to enter the mold. These fluxes are crucial both to prevent reoxidation through a complete covering of the rising steel and to insulate the steel and prevent premature solidification and skulling.
Presently two separate casting powders are applied in the production of each bottom poured ingot. First, a sealed bag of bottom pour flux is suspended in the mold approximately six to eighteen inches above the inlet for the molten steel. The bottom pour flux consists of chemical compounds which melt and spread rapidly across the surface of the molten steel. The molten slag coating the surface of the steel acts to create the correct menisus shape and prevents oxidation of steel surface. Additionally, the molten slag insulates the surface of the molten steel to slow solidification, and spreads a thin coat of homogenous glass between the mold and the molten steel to allow constant heat transfer and solidification and thus lessen thermally induced stresses and resultant cracking. Further the flux absorbs impurities such as deoxidation and reoxidation products and refractory particles. The bag containing the bottom pour flux burns upon the introduction of the molten steel into the mold thus automatically releasing the flux.
Although traditional bottom pour fluxes are crucial for efficient production of steel ingots they are not sufficient. In order to assure the surface quality of the ingots and maximizing yield by avoiding "pipe" (i.e. shrinkage and segregation), an additional layer of insulative material must be added immediately after the molten steel has filled the mold and entered "hot top" region. This material is referred to as "hot topping compound." Without the addition of hot topping compound, the molten steel would freeze in the hot top, thus not providing liquid steel to feed the shrinkage cavity (i.e. pipe) formed due to ingot solidification. The result would be to discard an entire segment of the steel ingot causing reduction in yield.
However, the application of hot topping compounds is not without its detractions. Adding hot topping compound is cumbersome, labor intensive, environmentally disruptive, and may contaminate the ingot steel.
Personnel, who could be better utilized elsewhere, must be stationed on the pouring platform above the molds to distribute the hot topping compound bags onto the molten steel. In addition to the cost of personnel, this process has two serious drawbacks. First, the pouring of the fine grained hot topping compound some two to ten feet onto the top of the powdery flux layer generates extensive clouds of environmentally harmful dust and smoke. Second, the addition of hot topping compound has been associated with a condition known as "core of debris." Core of debris occurs when the chilling effect of the hot topping compound causes steel to solidify around refractory inclusions which then sink into and contaminate the steel ingots.
In light of the foregoing, it is a primary object of the present invention to create one casting powder which provides the benefits of both a bottom pour flux and a hot topping compound.
It is a further object of the present invention to provide a one-step casting powder which is automatically dispensed through the bag suspension-burn method or a board presently used to dispense bottom pour fluxes.
It is an additional object of the present invention to provide a one-step casting powder which is economic to produce and use, entails little environmental risk, and does not contribute to ingot contamination.